- Email: [email protected]
Development of ZrF4-containing CsF–AlF3 flux for brazing 5052 aluminium alloy with Zn–Al filler metal
Materials and Design 90 (2016) 610–617
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Development of ZrF4-containing CsF–AlF3 flux for brazing 5052 aluminium alloy with Zn–Al filler metal Bing Xiao a, Dongpo Wang a,b, Fangjie Cheng a,b,⁎, Ying Wang a,b a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 12 September 2015 Received in revised form 22 October 2015 Accepted 6 November 2015 Available online 7 November 2015 Keywords: Aluminium alloy Brazing and soldering Microstructure Oxide-film removal
a b s t r a c t Intermediate-temperature brazing of the 5052 aluminium alloy was conducted using Zn–xAl (x = 8, 15, and 22 wt.%) filler metals with a ZrF4-containing CsF–AlF3 flux developed in this study. The microstructure and mechanical properties of the brazed joints as well as the oxide-film removal behaviour during brazing were analysed. The results show that the ZrF4-containing CsF–AlF3 flux improved the wettability of the Zn–Al filler metals on the surface of the 5052 aluminium alloy. Because the Zr produced by the chemical reaction between Zr4+ and the Al substrate did not block the continued occurrence of this reaction under the oxide film, the activity of the flux was enhanced. The 5052 aluminium alloy was brazed successfully with Zn–15Al and Zn–22Al filler metals and CsF–AlF3 flux containing 4–6 mol% ZrF4. Microstructural observations of the brazing seam showed that it was composed of the α-Al phase, eutectoid structure, β-Zn phase, and a Zn–Al eutectic structure. The strength of the Zn–15Al brazing joint was higher than that of the Zn–22Al brazing joint, because the relatively low heating temperature used during brazing with the Zn–15Al filler metal resulted in reduced softening of the base metal. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The brazeability of the 5000 series Al alloys containing more than 2.0 wt.% Mg is poor because a Mg-enriched oxide film, which is difficult to remove, is formed on the surface of the alloys during brazing [1–4]. Further, because many 5000 series Al alloys have relatively low solidus temperatures (e.g., 574 °C for the 5083 Al alloy), they must be brazed at an intermediate-temperature below 550 °C [5–6].. Flux is often used to remove an oxide film formed on the surface. Flux-assisted brazing is widely used in the industry for joining of Al alloys because it provides convenient brazing operation conditions and stable joint properties. Thus, choosing appropriate fluxes to remove the surface oxide film is essential to brazing. Among the fluxes used in the brazing of Al alloys, the CsF–AlF3 flux is particularly beneficial to intermediate-temperature brazing because it is non-corrosive and has a low melting temperature of 471 °C; this flux has been extensively researched and applied in the brazing of Al alloys over the last decade [7–9]. For Al alloys containing less than 2.0 wt.% Mg (e.g., the 1060, 3003, 4032, and 6061 Al alloys), the oxide film formed on the surface is primarily composed of the Al2O3 phase during brazing, and the pure CsF–AlF3 flux can remove the oxide film. In contrast, for alloys ⁎ Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.matdes.2015.11.025 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
containing more than 2.0 wt.% Mg (e.g., the 5052, and 5083 Al alloys), the surface oxide film is mainly composed of MgO and MgAl2O4, and the pure CsF–AlF3 flux is unable to remove the oxide film [1,4].. The addition of activating agents in the flux can reduce the interfacial tension between the molten filler metal and the base metal, thus facilitating oxide-film removal. Yang et al. [10] brazed 6061 Al alloy and 304 stainless steel using a Zn–Al filler metal by applying CsF–AlF3 flux containing a small amount of RbF to remove the oxide film. Xue et al. [11] reported that the addition of ZnF2 in the CsF–AlF3 flux improved its ability to remove the oxide film formed on Al–Li alloys during brazing. In our previous work [12], the 5052 Al alloy was brazed using Zn– Al filler metals and CsF–AlF3 flux containing ZnCl2; however, Cl− is corrosive to the brazed joint. Thus, although some fluxes containing activating agents have been developed, non-corrosive modified fluxes that can be applied to the brazing of the 5000 series Al alloys are lacking. It has been known that Zr4+ can dissolve in the fluoride fluxes and probably improve the activity of the flux [13], but the forms in which it would be added and its effects on the removal of the surface oxide film in the 5000 series Al alloys are unclear. In the present study, ZrF4 was selected as the activating agent and a novel CsF–AlF3 flux containing ZrF4 was developed. The 5052 Al alloy, a typical 5000 series Al alloy, was then brazed using the developed flux. Three typical Zn–Al hypereutectic filler metals (i.e., Zn–8Al, Zn–15Al, and Zn–22Al) were used in the brazing because of their moderate brazing temperature, high corrosion resistance, and superior mechanical properties [14–16]. The
B. Xiao et al. / Materials and Design 90 (2016) 610–617 Table 1 Chemical composition and tensile strength of 5052 aluminium alloy. Material Compositions (wt.%) Al 5052
Si
Fe
Cu
Mn
Mg Zn
Bal. 0.25 0.40 0.10 0.10 2.6
Cr
Tensile strength, σ/MPa
0.10 0.15–0.35 195
Table 2 Solidus (Ts) and liquidus temperatures(TL) of Zn–Al filler metals. Filler metal
Ts/°C
TL/°C
Zn–8Al Zn–15Al Zn–22Al
385.8 388.2 451.4
421.0 446.6 488.2
microstructures and mechanical properties of the brazed joints were analysed. Additionally, the oxide-film removal mechanism by ZrF4containing CsF–AlF3 flux was investigated.
2. Experimental procedure The fluxes were prepared by solution synthesis. Based on the binary phase diagram of CsF–AlF3 [13], the e5 eutectic consisting of 58 mol% CsF and 42 mol% AlF3 was selected as the flux matrix; its theoretic melting point is 471 °C. The added concentrations of ZrF4 in the CsF–AlF3 flux matrix were 2 mol%, 4 mol%, 6 mol%, and 8 mol%.
611
The starting materials were Al(OH)3, anhydrous Cs2CO3, 40 wt.% HF solution and ZrF4. The preparation proceeded as follows. A specified amount of Al(OH)3 was acidified with excess aqueous HF containing an appropriate amount of ZrF4 until a clear solution was obtained. The Cs2CO3 solution was then added in a dropwise manner. The prepared blend was gradually heated to 100 °C until it was dry. The dry blend was then ground repeatedly until a homogeneous flux powder was obtained.. The base metal used in this work was the 5052 Al alloy, whose composition and tensile strength are listed in Table 1. Table 2 lists the solidus (Ts) and liquidus temperatures (TL) of the Zn–Al filler metals used in this work, which were determined by differential thermal analysis. The wetting performances of Zn–Al filler metals on 5052 Al alloy were examined by spreading tests. The 5052 Al-alloy base metals were processed into plates with dimensions of 40 mm × 40 mm × 2 mm, and then cleaned with a 15 wt.% NaOH solution and a 1:3 HNO3 solution successively before tests. Pure CsF–AlF3 flux, CsF–AlF3 flux containing 0.5 mol% KCl and CsF–AlF3–xZrF4 (x = 2, 4, 6, 8 mol%) flux were used. The spreading tests were performed in a resistance furnace according to China's National Standard GB/T 113642008. After the furnace was heated to 540 °C, the 5052 Al-alloy plates with the filler metal and flux on its surface were placed in the furnace quickly and then held for 10 min at 540 °C before being cooled to room temperature.. Brazing experiments were performed on the 5052 Al-alloy plates with dimensions of 50 mm × 20 mm × 2 mm. The Al alloy plates were pre-cleaned and lapping brazing joints with a lap length of 3–4 mm were employed, as illustrated schematically in Fig. 1(a). The brazing was conducted using the prepared CsF–AlF3 flux containing 6 mol% ZrF4 and the Zn–xAl (x = 8, 15, 22 wt.%) filler metals. The brazing temperature was 510 °C and holding time was 10 min when the Zn–8Al and Zn–15Al filler metals were used. The brazing temperature was 540 °C
Fig. 1. (a) Schematic illustration of the brazing joint; (b) shape and size of the 5052 Al-alloy samples for tensile test.
612
B. Xiao et al. / Materials and Design 90 (2016) 610–617
and holding time was 10 min when the Zn–22Al filler metal was used. The brazing procedure was the same as that in the spreading tests.. The brazed joints were etched with 4% HCl alcohol solution for 7–8 s after grinding and polishing. The microstructure was observed using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS). The strength of the brazed joints was tested on a SANS electromechanical universal testing machine according to China's National Standard GB/T 11363-2008. The softening effect of the 5052 Al alloy during the brazing process was investigated by heating the 5052 Alalloy samples at temperatures of 510 °C and 540 °C for 10 min before cooling to room temperature. The tensile tests were then conducted on the heat-treated 5052 Al-alloy samples. The shape and size of the 5052Al-alloy samples are shown in Fig. 1(b).
The action of the fluxes on the 5052 Al-alloy surface was observed in order to clarify the removal mechanism of the surface oxide-film in Mgcontaining Al alloys. First, the 5052 Al-alloy plates (diameter: 6 mm; thickness: 2 mm) were ground and polished. Next, a small amount of the CsF–AlF3 flux containing 6 mol% ZrF4 and that containing 0.5 mol% KCl were placed on the surface of the 5052 Al-alloy plates. The plates were then heated to 540 °C, and held for 10 min before they were cooled to room temperature. The cooled plates were observed using an OM and SEM equipped with an EDS. Based on the observation results, the oxidefilm removal mechanism by ZrF4-containing CsF–AlF3 flux was analysed.
3. Results and discussion 3.1. Wetting performances of filler metals on 5052 Al alloy The spreading tests showed that all three Zn–Al filler metals did not spread on the surface of the 5052 Al alloy when using the pure CsF–AlF3 flux and the CsF–AlF3 flux containing 0.5 mol% KCl, which indicates that the oxide film was not removed. The CsF–AlF3 flux can remove the γAl2O3 oxide film formed on the surface of pure aluminium by reacting
Fig. 2. Spreading test results of Zn–Al filler metals on the 5052 Al alloy: (a) Relationship between ZrF4 content and spreading areas; typical spreading images with (b) pure CsF– AlF3 flux and (c) CsF–AlF3 flux containing 4 mol% ZrF4.
Fig. 3. Macromorphology of the joint brazed using Zn–15Al filler metal: (a) appearance of the joint, (b) cross-section of the joint.
B. Xiao et al. / Materials and Design 90 (2016) 610–617
with it and then dissolving it during brazing [11]. However, pure CsF– AlF3 flux could not dissolve MgO and MgAl2O4, which are the main phases of the oxide film on the 5052 Al alloy during brazing. Further, a small amount of KCl was soluble in the fluoride flux, but it did not participate in the oxide-film removal process [17–18]. Therefore, it is difficult to remove the oxide film on the 5052 Al alloy with either CsF–AlF3 flux or CsF–AlF3 flux containing 0.5 mol% KCl. Owing to the use of the ZrF4-containing CsF–AlF3 flux, the Zn–Al filler metals spread over the alloy surface. Fig. 2 shows the spreading areas of 0.2 g Zn–Al filler metals on the 5052 Al alloy when using CsF– AlF3–xZrF4 flux (x = 2, 4, 6, 8 mol%). All spreading areas shown in Fig. 2a were calculated as the average of values obtained from three samples. Two typical spreading images of Zn–Al filler metals with pure CsF–AlF3 flux and CsF–AlF3 flux containing 4 mol% ZrF4 are also shown in Fig. 2. The spreading area of the Zn–Al filler metals increased as the ZrF4 concentration was increased from 2 to 6 mol%. The filler metals had the largest areas when the concentration was in the range of 4–6 mol %, which yielded the highest activity of the flux. However, the spreading areas decreased when the concentration was increased up to 8 mol%. These results indicated that ZrF4 could act as an activating agent in the CsF–AlF3 flux. The activating agent took effect via its chemical reactions with the base metal and with the molten filler metal. The reactions
613
reduced the interfacial tension between the molten filler metal and the base metal, thus improving the wetting between them. As the concentration of ZrF4 was increased from 2 to 6 mol%, the chemical reactions were enhanced and the wetting of the filler metals on the Al alloy was improved. However, with excessive addition of the activating agent, some impurities in the flux such as Si4+ and Fe2+, which originated from the starting materials (e.g., ZrF4,Cs2CO3 etc.), would block the wetting of the filler metals on the Al alloy, thus decreasing the spreading areas. The appropriate concentrations of ZrF4 in CsF–AlF3 flux were determined to be within 4–6 mol%. Fig. 2 also shows that when the same amount of flux is used, the spreading area increases with increasing Al content in the filler metals. The wetting performances of the filler metal were affected by the fluidity of the filler metal and the interaction between the filler metal and the base metal. Owing to the large solubility of Zn in Al, the molten Zn–Al filler metal diffused into the intergranular locations of the Al alloy as it flowed forward, which decreased the fluidity of the filler metal on the Al alloy [13]. With increasing Al content in the filler metal, the diffusion of the filler metal into the Al alloy was weakened, increasing its fluidity. Our experiments also showed that when using ZrF4-containing CsF– AlF3 flux, Zn–Al filler metals have the same wetting performances on other typical 5000 series Al alloys such as the 5083 Al alloy (Mg content: 4.0–4.9 wt.%) and the 5A06 Al alloy (Mg content: 5.8–6.8 wt.%). Thus, ZrF4-containing CsF–AlF3 flux is effective in removing oxide films on 5000 series Al alloys..
3.2. Microstructure of the brazed joints The brazing experiments showed that sound brazed joints were obtained using Zn–22Al and Zn–15Al filler metals by applying CsF–AlF3 flux containing 4–6 mol% ZrF4. These filler metals exhibited excellent clearance fill ability on the 5052 Al alloy. Conversely, brazing with the Zn–8Al filler metal did not achieve joining of the 5052 Al alloy because of its bad fluidity. Fig. 3 shows the typical macromorphology of the joint brazed with Zn–15Al filler metal. Fig. 4 shows the microstructures of the brazing seams. Table 3 lists the EDS results of spot analysis of the points in Fig. 4 marked using alphabets. The joints brazed using Zn–22Al and Zn–15Al filler metals consisted of four phases: α-Al phase (A and E regions), eutectoid structure (B and F regions), β-Zn phase (C and G regions) and Zn–Al eutectic structure (D and H regions). This structure is in agreement with that in the joints of other Al alloys brazed using Zn–Al hypoeutectic filler metals [6,19–20]. It should be noted that the phases in the brazed joints were not completely in agreement with the equilibrium phase diagram of the Zn–Al alloy (e.g. eutectic structure in the Zn–22Al brazing joint). This is attributed to the nonequilibrium solidification of the filler during cooling from the peak temperature in brazing. Fig. 5 shows the microstructures and EDS results of element line scanning at the cross-sectional interfaces of the brazed joints. As seen
Table 3 EDS analysis results of the points in Fig. 4. Filler metal
Point
Zn
Al
Mg
Zn–22Al
A B C D E F G H
65.17 75.36 98.60 96.50 68.29 77.92 98.59 95.77
34.75 24.57 1.29 3.45 31.58 22.08 1.35 4.21
0.08 0.07 0.11 0.11 0.13 0.09 0.06 0.02
Zn–15Al Fig. 4. Microstructures of the brazing seam: (a) Zn–22Al, (b) Zn–15Al; the alphabets indicate the points used in the EDS spot analysis.
614
B. Xiao et al. / Materials and Design 90 (2016) 610–617
in the figure, only a small amount of Mg dissolved into the brazing seam and the corresponding new phase, such as MgZn2, did not appear. The widths of the diffusing zone in the joints brazed by Zn–22Al and Zn– 15Al filler metals are 12.10 μm and 9.25 μm, respectively. The increase in the width could be explained by the diffusion behaviour. According to the Arrhenius equation, the diffusion coefficient of the elements in the diffusing zone increases with an increase in the brazing temperature. Because the brazing temperature when using Zn–22Al is higher than that when using Zn–15Al in this work, the diffusion zone in the joint brazed using Zn–22Al filler metal is wider, which is in agreement with the results from the literature [19].
during the heat treatment owing to the recovery of the cold-worked 5052 Al alloy, and the relatively low heating temperature resulted in reduced softening of the base metal. The tensile strength of the joint brazed with Zn–15Al was 166.9 MPa and that of the joint brazed with Zn–22Al was 157.5 MPa. The strength of the brazed joints was slightly lower than that of the heat-treated base metals, probably because of the stress concentration in the transition location. The results of the mechanical properties tests indicated that the strength of the brazed joint was mainly affected by the softening of the base metal.
3.3. Mechanical properties of the brazed joints
Fig. 7 shows the surface morphologies of the 5052 Al alloy samples after the action of CsF–AlF3 flux containing 0.5 mol% KCl and that containing 6 mol% ZrF4. Because chloride is soluble in water, the sample with CsF–AlF3 flux containing 0.5 mol% KCl was washed with deionized water before observation. Table 4 lists the EDS analysis results of region A and B in Fig. 7. Fig. 7a–b and the above-mentioned results of the spreading tests show that the CsF–AlF3 flux containing 0.5 mol% KCl could not remove the oxide film, but caused many cracks on the surface (oxide film) of the sample through the combined effect of heating and the chemisorptions of F− ions in the flux during brazing. Cracks are easily formed due to the difference in the thermal expansion coefficients of the Al substrate and oxide film (αMgO = 13.8 × 10−6/°C; αAl = 28.5 × 10−6/°C)
Fig. 6 shows the results of the mechanical properties tests. The strength values in Fig. 6a are calculated as the average of values obtained for three samples. The joints brazed with Zn–22Al and Zn–15Al filler metals both fractured in the transition of the base metal and the brazing seam, as shown in Fig. 6b. For easy comparison, the strengths of the brazed joints in Fig. 6a are converted into tensile strengths. The tensile strength of the 5052 Al-alloy base metals deceased with an increase in brazing temperature. When the temperature was increased from 510 °C to 540 °C, the tensile strength of the 5052 Al alloy reduced from 170.8 MPa to 162.4 MPa, whereas the tensile strength of the un-heated 5052 Al alloy was 195 MPa. The base metal was softened
3.4. Oxide-film removal mechanism of ZrF4-containing CsF–AlF3 flux
Fig. 5. (a) SEM images of brazed interface with Zn–22Al filler metal; (b) metal element line scanning results of line 1 in (a); (c) SEM images of brazed interface with Zn–15Al filler; (d) metal element line scanning results of line 2 in (c).
B. Xiao et al. / Materials and Design 90 (2016) 610–617
in the heating process. In addition, F− can be chemisorbed in the oxide film because of the relatively small radius of F− ions in the flux, which also promoted the cracking of the oxide film [18].These cracks acted as entrance points for the flux. As a result, the molten flux flowed through the cracks and permeated into the film. The EDS results in Table 4 also indicate that the white residual flux flowed in the cracks. Because KCl did not participate in the oxide-film removal process, it can be inferred that a similar change would occur on the surface of the 5052 Al alloy by the action of pure CsF–AlF3 flux. As shown in Fig. 7c and d, a few black particles can be seen in the white ZrF4-containing flux residue. According to the reaction principle of the activating agents [13], Zr4+ permeates the film with the molten flux and chemically reacts with the Al substrate under the oxide film according to the equation 4Al + 3Zr4+ = 3Zr↓ + 4Al3+, which is driven by the more positive electrode potentials of Zr than that of Al. The above chemical reaction can destroy the connection between the oxide film and the Al substrate and thus loosen the film. Based on the above reaction and the EDS analysis results of region B in Fig. 7d, it can be inferred that the black substance was in fact Zr produced as a result of the reaction between Zr4+ and the Al substrate, while the Al3+ was produced and dissolved in the flux. Wetting between the molten filler metal and the Al alloy occurs only if the surface oxide film has been removed (or loosened), and the spreading of the filler metal can push the loosened oxide film away [13,18]. Fig. 8 is the cross-sectional OM image of the spreading sample using the Zn–15Al filler metal and CsF–AlF3 flux containing 6 mol% ZrF4. The spreading of the filler metal in the OM image indicates that
615
the reaction of Zr4 + and the Al substrate has loosened the surface oxide film in the spreading area. It also can be seen that the filler metal diffused into the base metal as it spread forwards. This means that the spreading and diffusion of the filler metal have pushed the loosened oxide film away as the molten filler metal crept along the Al surface. The activating effect of Zr4+ in intermediate-temperature brazing is an interesting phenomenon. Si4+, Ge4+, and Zr4+ are generally considered to be in the same class of activating agents. When Si4+ acts as an activating agent in fluoride flux, the Si produced by the reaction between Si4+ and the Al substrate is deposited on the surface of Al alloy because of the high diffusion coefficient of Si in Al [13,21]. Thus, the deposited Si blocks the wetting of the filler metal and the base metal at intermediate temperatures. The wetting of the filler metal would be improved only if liquid alloying of Si and the Al substrate occurs as the brazing temperature is above the eutectic temperature of the Al–Si alloy (577 °C). In contrast, ZrF4-containing CsF–AlF3 flux in this work significantly improved the wetting of Zn–Al filler metals on 5000 series Al alloys although the liquid alloying of the produced Zr and the Al substrate did not occur during the intermediate-temperature brazing according to the phase diagram of Al–Zr. The activity of ZrF4-containing CsF–AlF3 flux is attributed to the characteristics of the produced Zr. According to the Arrhenius equation, Q ; D ¼ D0 exp − RT where Q is the activating energy, R is the gas constant and T is the absolute temperature. The diffusion coefficient of Si in Al is Dsi → Al = 2.49 × 10−13 m2/s, whereas the diffusion coefficient of Zr in Al is DZr→ −18 m2/s at 540 °C. The diffusion coefficient of Zr in Al is Al = 8.35 × 10 much smaller than that of Si in Al. When Zr was produced by the chemical reaction of Zr4+ and the Al substrate, most of Zr did not diffuse towards the surface of the Al alloy. On the other hand, the EDS results in Table 4 show that O was detected in the flux residue in the 5052 Al-alloy sample, which indicates that O was absorbed by the flux when the molten flux flowed on the surface of the Al alloy. The binding force between Zr and O, which is known to be strong [22–23], drove the diffusion of Zr into the flux and its spread over the alloy surface together with the flux, as shown in Fig. 7c. Thus, the permeation of the molten flux was not blocked, and the continued occurrence of the reaction between Zr4+ and the Al substrate loosened the oxide film. Therefore, the ZrF4-containing CsF–AlF3 flux exhibited excellent activity. The oxide-film removal process by ZrF4-containing CsF–AlF3 flux can be summarized as follows. A model is presented in Fig. 9 to illustrate this process. (a) The oxide film on the surface of the 5052 Al alloys cracked during brazing. (b) The molten flux permeated the oxide film through the cracks and then Zr4 + chemically reacted with the Al substrate. The produced Zr diffused into the flux and spread with it, and this did not block the chemical reaction between Zr4+ and the Al substrate under the oxide film. (c) The continued occurrence of the chemical reaction destroyed the connection between the oxide film and the Al substrate, thus loosening the film. When the molten filler metal crept along the Al surface, the spreading and diffusion of the filler metal pushed and removed the loosened oxide film.
4. Conclusions Fig. 6. (a) Tensile strengths of heat-treated 5052 Al alloy and brazed joints; (b) typical fracture images of the heated-treated 5052 Al alloy and the brazed sample.
In this study, ZrF4-containing CsF–AlF3 flux was developed and used in the brazing of 5052 Al alloy. The conclusions obtained are as follows.
616
B. Xiao et al. / Materials and Design 90 (2016) 610–617
Fig. 7. Local OM image (a) and SEM image (b) of the 5052 Al alloy after the action of CsF–AlF3–0.5 mol% KCl flux; Local OM image (c) and SEM image (d) of the 5052 Al alloy after the action of CsF–AlF3–6 mol% ZrF4 flux; the alphabets indicate regions used in the EDS analysis.
(a) With the aid of the developed ZrF4-containing CsF–AlF3 flux, a sound joint was obtained using Zn–22Al and Zn–15Al filler metals. The strength of the Zn–15Al brazing joint was higher than that of the Zn–22Al brazing joint because the relatively low heating temperature used during brazing with the Zn–15Al filler metal resulted in reduced softening of the base metal. (b) Microstructural observations of the brazing seam show that it was composed of the α-Al phase, eutectoid structure, β-Zn phase, and Zn–Al eutectic structure. Only a small amount of Mg was found to dissolve in the brazing seam during brazing. (c) The wettability of the Zn–Al hypoeutectic filler metals on the surface of the 5000 series Al alloys was improved by using ZrF4containing CsF–AlF3 flux. The appropriate concentration of ZrF4 in CsF–AlF3 flux was determined to be 4–6 mol%. Because the Zr produced by the chemical reaction between Zr4+ and the Al substrate did not block the continued occurrence of this reaction, the activity of the flux was enhanced.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51275351), and the Natural Science Foundation of Tianjin, China (Grant No. 13JCZDJC33500).
Table 4 EDS analysis results of region A and B in Fig. 7. Region
F
Al
Mg
Cs
Zr
O
K
Cl
A B
17.52 12.10
30.25 18.38
0.75 0.37
42.19 46.06
9.21
7.75 13.88
1.35 –
0.20 –
Fig. 8. Cross-sectional image of local morphology of spreading sample.
B. Xiao et al. / Materials and Design 90 (2016) 610–617
617
Fig. 9. A model of the oxide-film removal process: (a) oxide film was cracked, (b) oxide film was loosened, (c) oxide film was pushed away.
References [1] F.J. Cheng, H.W. Zhao, Y. Wang, B. Xiao, J.F. Yao, Evolution of surface oxide film of typical aluminium alloys under medium-temperature brazing process, Trans Tianjin Univ 20 (1) (2014) 54–59. [2] E. Panda, L.P.H. Jeurgens, G. Richter, E.J. Mittemeijer, The amorphous to crystalline transition of ultrathin (Al, Mg)-oxide films grown by thermal oxidation of AlMg alloys: a high-resolution transmission electron microscopy investigation, J. Mater. Res. 25 (5) (2010) 871–879. [3] D.J. FIELD, G.M. SCAMANS, E.P. BUTLER, The high temperature oxidation of Al–4.2 wt pct Mg alloy, Metall trans A 18 (3) (1987) 463–472. [4] W. Takehiko, A. Haruhiko, Y. Atsushi, Brazing of A5056 aluminium alloy with the aid of ultrasonic vibration using Ag filler metal, J Japan Weld Soc 26 (4) (2008) 259–263. [5] T. Nagaoka, Y. Morisada, M. Fukusumi, T. Takemoto, Joint strength of aluminium ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder, J. Mater. Process. Technol. 209 (2009) 5054–5059. [6] T. Nagaoka, Y. Morisada, M. Fukusumi, T. Takemoto, Selection of soldering temperature for ultrasonic-assisted soldering of 5056 aluminium alloy using Zn–Al system solders, J. Mater. Process. Technol. 211 (2011) 1534–1539. [7] W. Luo, L.T. Wang, Q.M. Wang, H.L. Gong, M. Yan, A new filler metal with low contents of Cu for high strength aluminium alloy brazed joints, Mater. Des. 63 (2014) 263–269. [8] W. Dai, S.B. Xue, J.Y. Lou, S.Q. Wang, Development of Al–Si–Zn–Sr filler metals for brazing 6061 aluminium alloy, Mater. Des. 42 (2012) 395–402. [9] F. Ji, S.B. Xue, W. Dai, Reliability studies of Cu/Al joints brazed with Zn–Al–Ce filler metal, Mater. Des. 42 (2012) 156–163. [10] J.L. Yang, S.B. Xue, P. Xue, Z.P. Lv, W. Dai, J.X. Zhang, Development of novel CsF–RbF– AlF3 flux for brazing aluminium to stainless steel with Zn–Al filler metal, Mater. Des. 64 (2014) 110–115. [11] S.B. Xue, L. Zhang, Z.J. Han, X. Huang, Reaction mechanism between oxide film on the surface of Al–Li alloy and CsF–AlF3 flux, Thans Nonferrous Met Soc China 18 (3) (2008) 121–125.
[12] B. Xiao, D.P. Wang, F.J. Cheng, Y. Wang, Oxide film on 5052 aluminium alloy: its structure and removal mechanism by activated CsF–AlF3 flux in brazing, Appl. Surf. Sci. 337 (2015) 208–215. [13] Q.Y. Zhang, H.S. Zhuang, Brazing and Soldering Manual, China Machine Press, Beijing, 2008. [14] Y.M. Zhang, L.J. Yang, X.D. Zeng, B.Z. Zheng, Z.L. Song, The mechanism of annealhardening phenomenon in extruded Zn–Al alloys, Mater. Des. 50 (2013) 223–229. [15] X.Q. Yan, S.X. Liu, W.M. Long, J.L. Huang, L.Y. Zhang, Y. Chen, The effect of homogenization treatment on microstructure and properties of ZnAl15 solder, Mater. Des. 45 (2013) 440–445. [16] Y. Xiao, H.J. Ji, M.Y. Li, J.Y. Kim, H. Kim, Microstructure and joint properties of ultrasonically brazed Al alloy joints using a Zn–Al hypereutectic filler metal, Mater. Des. 47 (2013) 717–724. [17] J. Hu, Q.Y. Zhang, Investigation of pseudo-ternary system AlF3–KF–KCl, Thermochim. Acta 404 (2003) 3–7. [18] Q.Y. Zhang, S.Q. Liu, A. Jian, Action of flux on the oxide film in aluminium brazing, Acta Metall. Sin. 21 (1) (1985) 35–39. [19] W. Dai, S.B. Xue, J.Y. Lou, Y.B. Lou, S.Q. Wang, Torch brazing 3003 aluminium alloy with Zn–Al filler metal, Trans Nonferrous Met Soc China 22 (2012) 30–35. [20] T.Y. Luan, W.B. Guo, J. Yu, Microstructure analysis of 1060 Al brazing with Zn–20Al solder, Welding 57 (7) (2014) 10. [21] Z.M. Wang, J.Y. Wang, L.P.H. Jeurgens, E.J. Mittemeijer, Thermodynamics and mechanism of metal-induced crystallization in immiscible alloy systems: experiments and calculations on Al/α-Ge and Al/α-Si bilayers, Phys. Rev. B 77 (4) (2008) 045424 -1–15. [22] W. Dang, J.S. Li, T.B. Zhang, H.C. Kou, Oxidation behaviour of Zr-containing Ti2AlNbbased alloy at 800 °C[J], Trans Nonferrous Met Soc China 25 (3) (2015) 783–790. [23] Y.H. Wong, K.Y. Cheong, Thermal oxidation and nitridation of sputtered Zr thin film on Si via N2O gas, J. Alloys Compd. 509 (35) (2011) 8728–8738.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Development of ZrF4-containing CsF–AlF3 flux for brazing 5052 aluminium alloy with Zn–Al filler metal Bing Xiao a, Dongpo Wang a,b, Fangjie Cheng a,b,⁎, Ying Wang a,b a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 12 September 2015 Received in revised form 22 October 2015 Accepted 6 November 2015 Available online 7 November 2015 Keywords: Aluminium alloy Brazing and soldering Microstructure Oxide-film removal
a b s t r a c t Intermediate-temperature brazing of the 5052 aluminium alloy was conducted using Zn–xAl (x = 8, 15, and 22 wt.%) filler metals with a ZrF4-containing CsF–AlF3 flux developed in this study. The microstructure and mechanical properties of the brazed joints as well as the oxide-film removal behaviour during brazing were analysed. The results show that the ZrF4-containing CsF–AlF3 flux improved the wettability of the Zn–Al filler metals on the surface of the 5052 aluminium alloy. Because the Zr produced by the chemical reaction between Zr4+ and the Al substrate did not block the continued occurrence of this reaction under the oxide film, the activity of the flux was enhanced. The 5052 aluminium alloy was brazed successfully with Zn–15Al and Zn–22Al filler metals and CsF–AlF3 flux containing 4–6 mol% ZrF4. Microstructural observations of the brazing seam showed that it was composed of the α-Al phase, eutectoid structure, β-Zn phase, and a Zn–Al eutectic structure. The strength of the Zn–15Al brazing joint was higher than that of the Zn–22Al brazing joint, because the relatively low heating temperature used during brazing with the Zn–15Al filler metal resulted in reduced softening of the base metal. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction The brazeability of the 5000 series Al alloys containing more than 2.0 wt.% Mg is poor because a Mg-enriched oxide film, which is difficult to remove, is formed on the surface of the alloys during brazing [1–4]. Further, because many 5000 series Al alloys have relatively low solidus temperatures (e.g., 574 °C for the 5083 Al alloy), they must be brazed at an intermediate-temperature below 550 °C [5–6].. Flux is often used to remove an oxide film formed on the surface. Flux-assisted brazing is widely used in the industry for joining of Al alloys because it provides convenient brazing operation conditions and stable joint properties. Thus, choosing appropriate fluxes to remove the surface oxide film is essential to brazing. Among the fluxes used in the brazing of Al alloys, the CsF–AlF3 flux is particularly beneficial to intermediate-temperature brazing because it is non-corrosive and has a low melting temperature of 471 °C; this flux has been extensively researched and applied in the brazing of Al alloys over the last decade [7–9]. For Al alloys containing less than 2.0 wt.% Mg (e.g., the 1060, 3003, 4032, and 6061 Al alloys), the oxide film formed on the surface is primarily composed of the Al2O3 phase during brazing, and the pure CsF–AlF3 flux can remove the oxide film. In contrast, for alloys ⁎ Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.matdes.2015.11.025 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
containing more than 2.0 wt.% Mg (e.g., the 5052, and 5083 Al alloys), the surface oxide film is mainly composed of MgO and MgAl2O4, and the pure CsF–AlF3 flux is unable to remove the oxide film [1,4].. The addition of activating agents in the flux can reduce the interfacial tension between the molten filler metal and the base metal, thus facilitating oxide-film removal. Yang et al. [10] brazed 6061 Al alloy and 304 stainless steel using a Zn–Al filler metal by applying CsF–AlF3 flux containing a small amount of RbF to remove the oxide film. Xue et al. [11] reported that the addition of ZnF2 in the CsF–AlF3 flux improved its ability to remove the oxide film formed on Al–Li alloys during brazing. In our previous work [12], the 5052 Al alloy was brazed using Zn– Al filler metals and CsF–AlF3 flux containing ZnCl2; however, Cl− is corrosive to the brazed joint. Thus, although some fluxes containing activating agents have been developed, non-corrosive modified fluxes that can be applied to the brazing of the 5000 series Al alloys are lacking. It has been known that Zr4+ can dissolve in the fluoride fluxes and probably improve the activity of the flux [13], but the forms in which it would be added and its effects on the removal of the surface oxide film in the 5000 series Al alloys are unclear. In the present study, ZrF4 was selected as the activating agent and a novel CsF–AlF3 flux containing ZrF4 was developed. The 5052 Al alloy, a typical 5000 series Al alloy, was then brazed using the developed flux. Three typical Zn–Al hypereutectic filler metals (i.e., Zn–8Al, Zn–15Al, and Zn–22Al) were used in the brazing because of their moderate brazing temperature, high corrosion resistance, and superior mechanical properties [14–16]. The
B. Xiao et al. / Materials and Design 90 (2016) 610–617 Table 1 Chemical composition and tensile strength of 5052 aluminium alloy. Material Compositions (wt.%) Al 5052
Si
Fe
Cu
Mn
Mg Zn
Bal. 0.25 0.40 0.10 0.10 2.6
Cr
Tensile strength, σ/MPa
0.10 0.15–0.35 195
Table 2 Solidus (Ts) and liquidus temperatures(TL) of Zn–Al filler metals. Filler metal
Ts/°C
TL/°C
Zn–8Al Zn–15Al Zn–22Al
385.8 388.2 451.4
421.0 446.6 488.2
microstructures and mechanical properties of the brazed joints were analysed. Additionally, the oxide-film removal mechanism by ZrF4containing CsF–AlF3 flux was investigated.
2. Experimental procedure The fluxes were prepared by solution synthesis. Based on the binary phase diagram of CsF–AlF3 [13], the e5 eutectic consisting of 58 mol% CsF and 42 mol% AlF3 was selected as the flux matrix; its theoretic melting point is 471 °C. The added concentrations of ZrF4 in the CsF–AlF3 flux matrix were 2 mol%, 4 mol%, 6 mol%, and 8 mol%.
611
The starting materials were Al(OH)3, anhydrous Cs2CO3, 40 wt.% HF solution and ZrF4. The preparation proceeded as follows. A specified amount of Al(OH)3 was acidified with excess aqueous HF containing an appropriate amount of ZrF4 until a clear solution was obtained. The Cs2CO3 solution was then added in a dropwise manner. The prepared blend was gradually heated to 100 °C until it was dry. The dry blend was then ground repeatedly until a homogeneous flux powder was obtained.. The base metal used in this work was the 5052 Al alloy, whose composition and tensile strength are listed in Table 1. Table 2 lists the solidus (Ts) and liquidus temperatures (TL) of the Zn–Al filler metals used in this work, which were determined by differential thermal analysis. The wetting performances of Zn–Al filler metals on 5052 Al alloy were examined by spreading tests. The 5052 Al-alloy base metals were processed into plates with dimensions of 40 mm × 40 mm × 2 mm, and then cleaned with a 15 wt.% NaOH solution and a 1:3 HNO3 solution successively before tests. Pure CsF–AlF3 flux, CsF–AlF3 flux containing 0.5 mol% KCl and CsF–AlF3–xZrF4 (x = 2, 4, 6, 8 mol%) flux were used. The spreading tests were performed in a resistance furnace according to China's National Standard GB/T 113642008. After the furnace was heated to 540 °C, the 5052 Al-alloy plates with the filler metal and flux on its surface were placed in the furnace quickly and then held for 10 min at 540 °C before being cooled to room temperature.. Brazing experiments were performed on the 5052 Al-alloy plates with dimensions of 50 mm × 20 mm × 2 mm. The Al alloy plates were pre-cleaned and lapping brazing joints with a lap length of 3–4 mm were employed, as illustrated schematically in Fig. 1(a). The brazing was conducted using the prepared CsF–AlF3 flux containing 6 mol% ZrF4 and the Zn–xAl (x = 8, 15, 22 wt.%) filler metals. The brazing temperature was 510 °C and holding time was 10 min when the Zn–8Al and Zn–15Al filler metals were used. The brazing temperature was 540 °C
Fig. 1. (a) Schematic illustration of the brazing joint; (b) shape and size of the 5052 Al-alloy samples for tensile test.
612
B. Xiao et al. / Materials and Design 90 (2016) 610–617
and holding time was 10 min when the Zn–22Al filler metal was used. The brazing procedure was the same as that in the spreading tests.. The brazed joints were etched with 4% HCl alcohol solution for 7–8 s after grinding and polishing. The microstructure was observed using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS). The strength of the brazed joints was tested on a SANS electromechanical universal testing machine according to China's National Standard GB/T 11363-2008. The softening effect of the 5052 Al alloy during the brazing process was investigated by heating the 5052 Alalloy samples at temperatures of 510 °C and 540 °C for 10 min before cooling to room temperature. The tensile tests were then conducted on the heat-treated 5052 Al-alloy samples. The shape and size of the 5052Al-alloy samples are shown in Fig. 1(b).
The action of the fluxes on the 5052 Al-alloy surface was observed in order to clarify the removal mechanism of the surface oxide-film in Mgcontaining Al alloys. First, the 5052 Al-alloy plates (diameter: 6 mm; thickness: 2 mm) were ground and polished. Next, a small amount of the CsF–AlF3 flux containing 6 mol% ZrF4 and that containing 0.5 mol% KCl were placed on the surface of the 5052 Al-alloy plates. The plates were then heated to 540 °C, and held for 10 min before they were cooled to room temperature. The cooled plates were observed using an OM and SEM equipped with an EDS. Based on the observation results, the oxidefilm removal mechanism by ZrF4-containing CsF–AlF3 flux was analysed.
3. Results and discussion 3.1. Wetting performances of filler metals on 5052 Al alloy The spreading tests showed that all three Zn–Al filler metals did not spread on the surface of the 5052 Al alloy when using the pure CsF–AlF3 flux and the CsF–AlF3 flux containing 0.5 mol% KCl, which indicates that the oxide film was not removed. The CsF–AlF3 flux can remove the γAl2O3 oxide film formed on the surface of pure aluminium by reacting
Fig. 2. Spreading test results of Zn–Al filler metals on the 5052 Al alloy: (a) Relationship between ZrF4 content and spreading areas; typical spreading images with (b) pure CsF– AlF3 flux and (c) CsF–AlF3 flux containing 4 mol% ZrF4.
Fig. 3. Macromorphology of the joint brazed using Zn–15Al filler metal: (a) appearance of the joint, (b) cross-section of the joint.
B. Xiao et al. / Materials and Design 90 (2016) 610–617
with it and then dissolving it during brazing [11]. However, pure CsF– AlF3 flux could not dissolve MgO and MgAl2O4, which are the main phases of the oxide film on the 5052 Al alloy during brazing. Further, a small amount of KCl was soluble in the fluoride flux, but it did not participate in the oxide-film removal process [17–18]. Therefore, it is difficult to remove the oxide film on the 5052 Al alloy with either CsF–AlF3 flux or CsF–AlF3 flux containing 0.5 mol% KCl. Owing to the use of the ZrF4-containing CsF–AlF3 flux, the Zn–Al filler metals spread over the alloy surface. Fig. 2 shows the spreading areas of 0.2 g Zn–Al filler metals on the 5052 Al alloy when using CsF– AlF3–xZrF4 flux (x = 2, 4, 6, 8 mol%). All spreading areas shown in Fig. 2a were calculated as the average of values obtained from three samples. Two typical spreading images of Zn–Al filler metals with pure CsF–AlF3 flux and CsF–AlF3 flux containing 4 mol% ZrF4 are also shown in Fig. 2. The spreading area of the Zn–Al filler metals increased as the ZrF4 concentration was increased from 2 to 6 mol%. The filler metals had the largest areas when the concentration was in the range of 4–6 mol %, which yielded the highest activity of the flux. However, the spreading areas decreased when the concentration was increased up to 8 mol%. These results indicated that ZrF4 could act as an activating agent in the CsF–AlF3 flux. The activating agent took effect via its chemical reactions with the base metal and with the molten filler metal. The reactions
613
reduced the interfacial tension between the molten filler metal and the base metal, thus improving the wetting between them. As the concentration of ZrF4 was increased from 2 to 6 mol%, the chemical reactions were enhanced and the wetting of the filler metals on the Al alloy was improved. However, with excessive addition of the activating agent, some impurities in the flux such as Si4+ and Fe2+, which originated from the starting materials (e.g., ZrF4,Cs2CO3 etc.), would block the wetting of the filler metals on the Al alloy, thus decreasing the spreading areas. The appropriate concentrations of ZrF4 in CsF–AlF3 flux were determined to be within 4–6 mol%. Fig. 2 also shows that when the same amount of flux is used, the spreading area increases with increasing Al content in the filler metals. The wetting performances of the filler metal were affected by the fluidity of the filler metal and the interaction between the filler metal and the base metal. Owing to the large solubility of Zn in Al, the molten Zn–Al filler metal diffused into the intergranular locations of the Al alloy as it flowed forward, which decreased the fluidity of the filler metal on the Al alloy [13]. With increasing Al content in the filler metal, the diffusion of the filler metal into the Al alloy was weakened, increasing its fluidity. Our experiments also showed that when using ZrF4-containing CsF– AlF3 flux, Zn–Al filler metals have the same wetting performances on other typical 5000 series Al alloys such as the 5083 Al alloy (Mg content: 4.0–4.9 wt.%) and the 5A06 Al alloy (Mg content: 5.8–6.8 wt.%). Thus, ZrF4-containing CsF–AlF3 flux is effective in removing oxide films on 5000 series Al alloys..
3.2. Microstructure of the brazed joints The brazing experiments showed that sound brazed joints were obtained using Zn–22Al and Zn–15Al filler metals by applying CsF–AlF3 flux containing 4–6 mol% ZrF4. These filler metals exhibited excellent clearance fill ability on the 5052 Al alloy. Conversely, brazing with the Zn–8Al filler metal did not achieve joining of the 5052 Al alloy because of its bad fluidity. Fig. 3 shows the typical macromorphology of the joint brazed with Zn–15Al filler metal. Fig. 4 shows the microstructures of the brazing seams. Table 3 lists the EDS results of spot analysis of the points in Fig. 4 marked using alphabets. The joints brazed using Zn–22Al and Zn–15Al filler metals consisted of four phases: α-Al phase (A and E regions), eutectoid structure (B and F regions), β-Zn phase (C and G regions) and Zn–Al eutectic structure (D and H regions). This structure is in agreement with that in the joints of other Al alloys brazed using Zn–Al hypoeutectic filler metals [6,19–20]. It should be noted that the phases in the brazed joints were not completely in agreement with the equilibrium phase diagram of the Zn–Al alloy (e.g. eutectic structure in the Zn–22Al brazing joint). This is attributed to the nonequilibrium solidification of the filler during cooling from the peak temperature in brazing. Fig. 5 shows the microstructures and EDS results of element line scanning at the cross-sectional interfaces of the brazed joints. As seen
Table 3 EDS analysis results of the points in Fig. 4. Filler metal
Point
Zn
Al
Mg
Zn–22Al
A B C D E F G H
65.17 75.36 98.60 96.50 68.29 77.92 98.59 95.77
34.75 24.57 1.29 3.45 31.58 22.08 1.35 4.21
0.08 0.07 0.11 0.11 0.13 0.09 0.06 0.02
Zn–15Al Fig. 4. Microstructures of the brazing seam: (a) Zn–22Al, (b) Zn–15Al; the alphabets indicate the points used in the EDS spot analysis.
614
B. Xiao et al. / Materials and Design 90 (2016) 610–617
in the figure, only a small amount of Mg dissolved into the brazing seam and the corresponding new phase, such as MgZn2, did not appear. The widths of the diffusing zone in the joints brazed by Zn–22Al and Zn– 15Al filler metals are 12.10 μm and 9.25 μm, respectively. The increase in the width could be explained by the diffusion behaviour. According to the Arrhenius equation, the diffusion coefficient of the elements in the diffusing zone increases with an increase in the brazing temperature. Because the brazing temperature when using Zn–22Al is higher than that when using Zn–15Al in this work, the diffusion zone in the joint brazed using Zn–22Al filler metal is wider, which is in agreement with the results from the literature [19].
during the heat treatment owing to the recovery of the cold-worked 5052 Al alloy, and the relatively low heating temperature resulted in reduced softening of the base metal. The tensile strength of the joint brazed with Zn–15Al was 166.9 MPa and that of the joint brazed with Zn–22Al was 157.5 MPa. The strength of the brazed joints was slightly lower than that of the heat-treated base metals, probably because of the stress concentration in the transition location. The results of the mechanical properties tests indicated that the strength of the brazed joint was mainly affected by the softening of the base metal.
3.3. Mechanical properties of the brazed joints
Fig. 7 shows the surface morphologies of the 5052 Al alloy samples after the action of CsF–AlF3 flux containing 0.5 mol% KCl and that containing 6 mol% ZrF4. Because chloride is soluble in water, the sample with CsF–AlF3 flux containing 0.5 mol% KCl was washed with deionized water before observation. Table 4 lists the EDS analysis results of region A and B in Fig. 7. Fig. 7a–b and the above-mentioned results of the spreading tests show that the CsF–AlF3 flux containing 0.5 mol% KCl could not remove the oxide film, but caused many cracks on the surface (oxide film) of the sample through the combined effect of heating and the chemisorptions of F− ions in the flux during brazing. Cracks are easily formed due to the difference in the thermal expansion coefficients of the Al substrate and oxide film (αMgO = 13.8 × 10−6/°C; αAl = 28.5 × 10−6/°C)
Fig. 6 shows the results of the mechanical properties tests. The strength values in Fig. 6a are calculated as the average of values obtained for three samples. The joints brazed with Zn–22Al and Zn–15Al filler metals both fractured in the transition of the base metal and the brazing seam, as shown in Fig. 6b. For easy comparison, the strengths of the brazed joints in Fig. 6a are converted into tensile strengths. The tensile strength of the 5052 Al-alloy base metals deceased with an increase in brazing temperature. When the temperature was increased from 510 °C to 540 °C, the tensile strength of the 5052 Al alloy reduced from 170.8 MPa to 162.4 MPa, whereas the tensile strength of the un-heated 5052 Al alloy was 195 MPa. The base metal was softened
3.4. Oxide-film removal mechanism of ZrF4-containing CsF–AlF3 flux
Fig. 5. (a) SEM images of brazed interface with Zn–22Al filler metal; (b) metal element line scanning results of line 1 in (a); (c) SEM images of brazed interface with Zn–15Al filler; (d) metal element line scanning results of line 2 in (c).
B. Xiao et al. / Materials and Design 90 (2016) 610–617
in the heating process. In addition, F− can be chemisorbed in the oxide film because of the relatively small radius of F− ions in the flux, which also promoted the cracking of the oxide film [18].These cracks acted as entrance points for the flux. As a result, the molten flux flowed through the cracks and permeated into the film. The EDS results in Table 4 also indicate that the white residual flux flowed in the cracks. Because KCl did not participate in the oxide-film removal process, it can be inferred that a similar change would occur on the surface of the 5052 Al alloy by the action of pure CsF–AlF3 flux. As shown in Fig. 7c and d, a few black particles can be seen in the white ZrF4-containing flux residue. According to the reaction principle of the activating agents [13], Zr4+ permeates the film with the molten flux and chemically reacts with the Al substrate under the oxide film according to the equation 4Al + 3Zr4+ = 3Zr↓ + 4Al3+, which is driven by the more positive electrode potentials of Zr than that of Al. The above chemical reaction can destroy the connection between the oxide film and the Al substrate and thus loosen the film. Based on the above reaction and the EDS analysis results of region B in Fig. 7d, it can be inferred that the black substance was in fact Zr produced as a result of the reaction between Zr4+ and the Al substrate, while the Al3+ was produced and dissolved in the flux. Wetting between the molten filler metal and the Al alloy occurs only if the surface oxide film has been removed (or loosened), and the spreading of the filler metal can push the loosened oxide film away [13,18]. Fig. 8 is the cross-sectional OM image of the spreading sample using the Zn–15Al filler metal and CsF–AlF3 flux containing 6 mol% ZrF4. The spreading of the filler metal in the OM image indicates that
615
the reaction of Zr4 + and the Al substrate has loosened the surface oxide film in the spreading area. It also can be seen that the filler metal diffused into the base metal as it spread forwards. This means that the spreading and diffusion of the filler metal have pushed the loosened oxide film away as the molten filler metal crept along the Al surface. The activating effect of Zr4+ in intermediate-temperature brazing is an interesting phenomenon. Si4+, Ge4+, and Zr4+ are generally considered to be in the same class of activating agents. When Si4+ acts as an activating agent in fluoride flux, the Si produced by the reaction between Si4+ and the Al substrate is deposited on the surface of Al alloy because of the high diffusion coefficient of Si in Al [13,21]. Thus, the deposited Si blocks the wetting of the filler metal and the base metal at intermediate temperatures. The wetting of the filler metal would be improved only if liquid alloying of Si and the Al substrate occurs as the brazing temperature is above the eutectic temperature of the Al–Si alloy (577 °C). In contrast, ZrF4-containing CsF–AlF3 flux in this work significantly improved the wetting of Zn–Al filler metals on 5000 series Al alloys although the liquid alloying of the produced Zr and the Al substrate did not occur during the intermediate-temperature brazing according to the phase diagram of Al–Zr. The activity of ZrF4-containing CsF–AlF3 flux is attributed to the characteristics of the produced Zr. According to the Arrhenius equation, Q ; D ¼ D0 exp − RT where Q is the activating energy, R is the gas constant and T is the absolute temperature. The diffusion coefficient of Si in Al is Dsi → Al = 2.49 × 10−13 m2/s, whereas the diffusion coefficient of Zr in Al is DZr→ −18 m2/s at 540 °C. The diffusion coefficient of Zr in Al is Al = 8.35 × 10 much smaller than that of Si in Al. When Zr was produced by the chemical reaction of Zr4+ and the Al substrate, most of Zr did not diffuse towards the surface of the Al alloy. On the other hand, the EDS results in Table 4 show that O was detected in the flux residue in the 5052 Al-alloy sample, which indicates that O was absorbed by the flux when the molten flux flowed on the surface of the Al alloy. The binding force between Zr and O, which is known to be strong [22–23], drove the diffusion of Zr into the flux and its spread over the alloy surface together with the flux, as shown in Fig. 7c. Thus, the permeation of the molten flux was not blocked, and the continued occurrence of the reaction between Zr4+ and the Al substrate loosened the oxide film. Therefore, the ZrF4-containing CsF–AlF3 flux exhibited excellent activity. The oxide-film removal process by ZrF4-containing CsF–AlF3 flux can be summarized as follows. A model is presented in Fig. 9 to illustrate this process. (a) The oxide film on the surface of the 5052 Al alloys cracked during brazing. (b) The molten flux permeated the oxide film through the cracks and then Zr4 + chemically reacted with the Al substrate. The produced Zr diffused into the flux and spread with it, and this did not block the chemical reaction between Zr4+ and the Al substrate under the oxide film. (c) The continued occurrence of the chemical reaction destroyed the connection between the oxide film and the Al substrate, thus loosening the film. When the molten filler metal crept along the Al surface, the spreading and diffusion of the filler metal pushed and removed the loosened oxide film.
4. Conclusions Fig. 6. (a) Tensile strengths of heat-treated 5052 Al alloy and brazed joints; (b) typical fracture images of the heated-treated 5052 Al alloy and the brazed sample.
In this study, ZrF4-containing CsF–AlF3 flux was developed and used in the brazing of 5052 Al alloy. The conclusions obtained are as follows.
616
B. Xiao et al. / Materials and Design 90 (2016) 610–617
Fig. 7. Local OM image (a) and SEM image (b) of the 5052 Al alloy after the action of CsF–AlF3–0.5 mol% KCl flux; Local OM image (c) and SEM image (d) of the 5052 Al alloy after the action of CsF–AlF3–6 mol% ZrF4 flux; the alphabets indicate regions used in the EDS analysis.
(a) With the aid of the developed ZrF4-containing CsF–AlF3 flux, a sound joint was obtained using Zn–22Al and Zn–15Al filler metals. The strength of the Zn–15Al brazing joint was higher than that of the Zn–22Al brazing joint because the relatively low heating temperature used during brazing with the Zn–15Al filler metal resulted in reduced softening of the base metal. (b) Microstructural observations of the brazing seam show that it was composed of the α-Al phase, eutectoid structure, β-Zn phase, and Zn–Al eutectic structure. Only a small amount of Mg was found to dissolve in the brazing seam during brazing. (c) The wettability of the Zn–Al hypoeutectic filler metals on the surface of the 5000 series Al alloys was improved by using ZrF4containing CsF–AlF3 flux. The appropriate concentration of ZrF4 in CsF–AlF3 flux was determined to be 4–6 mol%. Because the Zr produced by the chemical reaction between Zr4+ and the Al substrate did not block the continued occurrence of this reaction, the activity of the flux was enhanced.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51275351), and the Natural Science Foundation of Tianjin, China (Grant No. 13JCZDJC33500).
Table 4 EDS analysis results of region A and B in Fig. 7. Region
F
Al
Mg
Cs
Zr
O
K
Cl
A B
17.52 12.10
30.25 18.38
0.75 0.37
42.19 46.06
9.21
7.75 13.88
1.35 –
0.20 –
Fig. 8. Cross-sectional image of local morphology of spreading sample.
B. Xiao et al. / Materials and Design 90 (2016) 610–617
617
Fig. 9. A model of the oxide-film removal process: (a) oxide film was cracked, (b) oxide film was loosened, (c) oxide film was pushed away.
References [1] F.J. Cheng, H.W. Zhao, Y. Wang, B. Xiao, J.F. Yao, Evolution of surface oxide film of typical aluminium alloys under medium-temperature brazing process, Trans Tianjin Univ 20 (1) (2014) 54–59. [2] E. Panda, L.P.H. Jeurgens, G. Richter, E.J. Mittemeijer, The amorphous to crystalline transition of ultrathin (Al, Mg)-oxide films grown by thermal oxidation of AlMg alloys: a high-resolution transmission electron microscopy investigation, J. Mater. Res. 25 (5) (2010) 871–879. [3] D.J. FIELD, G.M. SCAMANS, E.P. BUTLER, The high temperature oxidation of Al–4.2 wt pct Mg alloy, Metall trans A 18 (3) (1987) 463–472. [4] W. Takehiko, A. Haruhiko, Y. Atsushi, Brazing of A5056 aluminium alloy with the aid of ultrasonic vibration using Ag filler metal, J Japan Weld Soc 26 (4) (2008) 259–263. [5] T. Nagaoka, Y. Morisada, M. Fukusumi, T. Takemoto, Joint strength of aluminium ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder, J. Mater. Process. Technol. 209 (2009) 5054–5059. [6] T. Nagaoka, Y. Morisada, M. Fukusumi, T. Takemoto, Selection of soldering temperature for ultrasonic-assisted soldering of 5056 aluminium alloy using Zn–Al system solders, J. Mater. Process. Technol. 211 (2011) 1534–1539. [7] W. Luo, L.T. Wang, Q.M. Wang, H.L. Gong, M. Yan, A new filler metal with low contents of Cu for high strength aluminium alloy brazed joints, Mater. Des. 63 (2014) 263–269. [8] W. Dai, S.B. Xue, J.Y. Lou, S.Q. Wang, Development of Al–Si–Zn–Sr filler metals for brazing 6061 aluminium alloy, Mater. Des. 42 (2012) 395–402. [9] F. Ji, S.B. Xue, W. Dai, Reliability studies of Cu/Al joints brazed with Zn–Al–Ce filler metal, Mater. Des. 42 (2012) 156–163. [10] J.L. Yang, S.B. Xue, P. Xue, Z.P. Lv, W. Dai, J.X. Zhang, Development of novel CsF–RbF– AlF3 flux for brazing aluminium to stainless steel with Zn–Al filler metal, Mater. Des. 64 (2014) 110–115. [11] S.B. Xue, L. Zhang, Z.J. Han, X. Huang, Reaction mechanism between oxide film on the surface of Al–Li alloy and CsF–AlF3 flux, Thans Nonferrous Met Soc China 18 (3) (2008) 121–125.
[12] B. Xiao, D.P. Wang, F.J. Cheng, Y. Wang, Oxide film on 5052 aluminium alloy: its structure and removal mechanism by activated CsF–AlF3 flux in brazing, Appl. Surf. Sci. 337 (2015) 208–215. [13] Q.Y. Zhang, H.S. Zhuang, Brazing and Soldering Manual, China Machine Press, Beijing, 2008. [14] Y.M. Zhang, L.J. Yang, X.D. Zeng, B.Z. Zheng, Z.L. Song, The mechanism of annealhardening phenomenon in extruded Zn–Al alloys, Mater. Des. 50 (2013) 223–229. [15] X.Q. Yan, S.X. Liu, W.M. Long, J.L. Huang, L.Y. Zhang, Y. Chen, The effect of homogenization treatment on microstructure and properties of ZnAl15 solder, Mater. Des. 45 (2013) 440–445. [16] Y. Xiao, H.J. Ji, M.Y. Li, J.Y. Kim, H. Kim, Microstructure and joint properties of ultrasonically brazed Al alloy joints using a Zn–Al hypereutectic filler metal, Mater. Des. 47 (2013) 717–724. [17] J. Hu, Q.Y. Zhang, Investigation of pseudo-ternary system AlF3–KF–KCl, Thermochim. Acta 404 (2003) 3–7. [18] Q.Y. Zhang, S.Q. Liu, A. Jian, Action of flux on the oxide film in aluminium brazing, Acta Metall. Sin. 21 (1) (1985) 35–39. [19] W. Dai, S.B. Xue, J.Y. Lou, Y.B. Lou, S.Q. Wang, Torch brazing 3003 aluminium alloy with Zn–Al filler metal, Trans Nonferrous Met Soc China 22 (2012) 30–35. [20] T.Y. Luan, W.B. Guo, J. Yu, Microstructure analysis of 1060 Al brazing with Zn–20Al solder, Welding 57 (7) (2014) 10. [21] Z.M. Wang, J.Y. Wang, L.P.H. Jeurgens, E.J. Mittemeijer, Thermodynamics and mechanism of metal-induced crystallization in immiscible alloy systems: experiments and calculations on Al/α-Ge and Al/α-Si bilayers, Phys. Rev. B 77 (4) (2008) 045424 -1–15. [22] W. Dang, J.S. Li, T.B. Zhang, H.C. Kou, Oxidation behaviour of Zr-containing Ti2AlNbbased alloy at 800 °C[J], Trans Nonferrous Met Soc China 25 (3) (2015) 783–790. [23] Y.H. Wong, K.Y. Cheong, Thermal oxidation and nitridation of sputtered Zr thin film on Si via N2O gas, J. Alloys Compd. 509 (35) (2011) 8728–8738.